FIG. 1 is a block diagram of a network structure of a universal mobile telecommunications system (UMTS). Referring to FIG. 1, the UMTS mainly includes a user equipment (UE), a UMTS terrestrial radio access network (UTRAN), and a core network (CN).
The UTRAN includes at least one radio network sub-system (RNS). The RNS includes one radio network controller (RNC) and at least one base station (Node B) managed by the RNC. At least one or more cells exist in one Node B.
FIG. 2 is an architectural diagram of a radio interface protocol between the UE and the UTRAN based on a 3GPP radio access network standard. Referring to FIG. 2, a radio interface protocol vertically includes a physical layer, a data link layer and a network layer. Horizontally the radio interface protocol includes a user plane for data information transfer and a control plane for signaling transfer. The protocol layers in FIG. 2 can be divided into a first layer (L1), a second layer (L2) and a third layer (L3) based on three lower layers of an open system interconnection (OSI) standard model widely known in the art of communication systems.
The respective layers in FIG. 2 are explained as follows. First, the physical layer (PHY) as the first layer offers an information transfer service to an upper layer using a physical channel. A medium access control (MAC) layer is located above the PHY. The physical layer (PHY) is connected to the (MAC) layer via a transport channel, wherein data is transferred between the MAC layer and the PHY via the transport channel. Moreover, data is transferred between different physical layers, and more particularly, between one physical layer of a transmitting side and another physical layer of a receiving side via the physical channel.
The MAC layer of the second layer offers a service to a radio link control (RLC) layer located above the MAC layer via a logical channel. The RLC layer of the second layer supports reliable data transfer and is operative in segmentation and concatenation of RLC service data units (SDUs) transferred from an upper layer.
A radio resource control (RRC) layer located at a lowest level of the third layer is defined in the control plane only and is associated with configuring, reconfiguring and releasing radio bearers (RBs) to be in charge of controlling the logical, transport and physical channels. In this case, the RB indicates a service offered to the second layer for transferring data between the UE and the UTRAN. And, the configuration of the RB indicates a process for regulating characteristics of protocol layers and channels necessary for offering a specific service and a process for setting their specific parameters and operational methods, respectively.
An RRC connection and signaling connection are explained in detail as follows. First, the UE makes an RRC connection to a UTRAN to initiate communication and makes a signaling connection to a CN. The UE exchanges UE dedicated control information with the UTRAN or CN via the RRC and signaling connections. FIG. 3 is a flowchart of a process for transmitting messages exchanged between the UE and the RNC for an RRC connection and transmitting an initial direct transfer (IDT) message for a signaling connection.
Referring to FIG. 3, for an RRC connection process, a UE transmits an RRC connection request message to an RNC (S11). The RNC then transmits an RRC connection setup message to the UE in response to the RRC connection request message (S12). Afterward, the UE transmits an RRC connection setup complete message to the RNC (S13). After successful completion of the above process, the RRC connection is established between the UE and the RNC. After the RRC connection is established, the UE initiates a process for establishing a signaling connection by transmitting an IDT message (S14).
A random access channel (RACH), which is a transport channel of an asynchronous mobile communication system, such as Wideband-CDMA (WCDMA), will be explained as follows. RACH is used in transmitting data having a short length in uplink. An RRC message, such as an RRC connection request message, a cell update message, a UTRAN registration area (URA) update message and the like may be transmitted via RACH. Logical channels, such as a common control channel (CCCH), a dedicated control channel (DCCH) or a dedicated traffic channel (DTCH) can be mapped to the transport channel RACH. Moreover, the transport channel RACH may be mapped to a physical channel, such as a physical random access channel (PRACH).
FIG. 4 is a diagram illustrating a transmission method of a physical channel PRACH in accordance with the related art. Referring to FIG. 4, an uplink physical channel PRACH is divided into a preamble part and a message part. The preamble part performs a power ramping function for adjusting a proper transmission power used for a message transmission. The preamble part also performs a function for preventing collisions between several user equipments (UEs). The message part transmits a MAC packet data unit (MAC PDU) delivered to a physical channel from a MAC layer.
If a UE's MAC layer instructs a UE's physical layer to make a PRACH transmission, the UE's physical layer first selects one access slot and one signature, and then transmits a PRACH preamble to a base station (Node B) in uplink. The preamble is transmitted during an access slot interval of 1.33 ms. One signature is selected from sixteen kinds of signatures and is then transmitted for a first predetermined length of the access slot.
If the UE transmits the preamble, the base station transmits a response signal via a downlink physical channel, such as an acquisition indicator channel (AICH). The response signal transmitted via the AICH in response to the preamble carries the signature selected by the preamble for a first predetermined length of an access slot corresponding to the former access slot having carried the preamble. In this case, the base station transmits an affirmative response (ACK) or a negative response (NACK) to the UE via the signature carried by the AICH.
If the UE receives the ACK, the UE transmits a message part having a length of 10 ms or 20 ms using an orthogonal variable spreading factor (OVSF) code corresponding to the transmitted signature. If the UE receives the NACK, the UE's MAC layer instructs the UE's physical layer to repeat the PRACH transmission after a proper duration. Meanwhile, if the UE fails to receive the response signal via the AICH corresponding to the transmitted preamble, the UE transmits a new preamble after the selected access slot with a power one step higher than that of the former preamble.
FIG. 5 is a structural diagram of a downlink physical channel AICH in accordance with the related art. Referring to FIG. 5, a downlink physical channel AICH transmits a 16-symbol signature SI (i=0 . . . 15) for an access slot having a 5,120-chip length. In this case, a UE selects an arbitrary signature SI from signatures S0 to S15 and transmits the signature for a first 4,096-chip length of the access slot while setting the rest of the access slot, having a 1,024-chip length, to a transmission power “OFF” interval for transmitting no symbol. Meanwhile, a preamble part of an uplink physical channel PRACH transmits a 16-symbol signature SI (i=0 . . . 15) for a first 4,096-chip length in a way similar to that shown in FIG. 4.
However, in the related art RACH transmission method, RACH transmission power should be raised when attempting to transmit a MAC PDU having a long length via RACH. Thus, the related art is problematic because cell coverage is reduced. Moreover, because limits are put on transmitting a message having a long length, an initial setup process using RACH is elongated.